THIS invention relates to a propeller and more particularly but not exclusively, to a propeller for use with inboard and outboard boat engines.
A propeller is a device that transmits power by converting rotational motion into thrust. A pressure differential is produced between forward and rear surfaces of the airfoil-shaped blade, and a fluid (such as air or water) is accelerated behind the blade, thus resulting in thrust required to drive a means of transport to which the propeller is attached. One specific type of propeller is a propeller for use as a means of propulsion in boat engines, whether outboard or inboard.
Many different propeller designs are known in the trade, and they all share some of the same design characteristics. A propeller comprises a plurality of blades extending radially outwardly from a central rotating hub. Each blade is shaped in the form of an airfoil having two opposite surfaces, being a blade face (which is the pressure side of the blade facing the stern), and the blade back (which is the suction side of the blade facing the bow). Each blade furthermore includes a leading edge, which is the edge of the propeller adjacent the forward end of the hub. The leading edge leads the blade into the flow when the propeller is providing forward thrust. The opposing edge is referred to as the trailing edge, and the radially outer zone extending between the leading edge and the trailing edge is referred to as the blade tip. The root of the blade is the fillet area in the region of transition between the blade surface and the hub periphery.
Blade surface area refers to the total surface area of the propeller blade. When a propeller rotates on a fixed axis for any period of time a centrifugal force creating a negative pressure on the blade back of each rotating blade draw water inwards, and when the oncoming blade face comes into contact with the inward flow of water the water is compressed. A positive pressure is therefore induced, and the water in this positive pressure zone then exerts a force against the adjacent body of water, resulting in thrust. Standard blade designs allow the inward flow of water to flow over the entire curvature of the blade back. This is believed to result in about 40% of the energy being wasted because on average only 60% of the negative pressure water mass is compressed by the blade face of an oncoming blade. Further energy is lost between the blade roots of each blade back, which fragments the flow of water when the positive pressure water mass collides with the negative pressure water mass. This disturbance affects the volume of water that gets displaced. It would obviously be beneficial if a way could be found to harness the wasted energy in order for the full potential energy of the water flow to be utilized.
It is accordingly an object of the invention to provide a propeller that will at least partially alleviate the above disadvantage.
It is also an object of the invention to provide a propeller having a secondary propulsion system which will aid optimizing the performance of the propeller.